Hicks A, Ohlendieck K, Göpel S O, Pette D
Faculty of Biology, University of Konstanz, Germany.
Am J Physiol. 1997 Jul;273(1 Pt 1):C297-305. doi: 10.1152/ajpcell.1997.273.1.C297.
To examine mechanisms underlying force reduction after the onset of chronic low-frequency (10 Hz) stimulation (CLFS), we exposed rabbit tibialis anterior muscles to various durations of CLFS. To follow changes in isometric contractile properties and electromyographic (EMG) activity, we studied stimulated and contralateral muscles during a terminal test at 10 Hz for 10 min. In addition, activities and protein amounts of the sarcoplasmic reticulum Ca(2+)-ATPase, content of Na(+)-K(+)-ATPase, and expression patterns of triad junction components were examined. Force output and EMG amplitude declined abruptly soon after the onset of stimulation, suggesting refractoriness of a large fiber population. Although twitch force and to a lesser extent EMG activity gradually recovered after stimulation for 6 days and longer, the muscles exhibited profoundly altered properties, i.e., enhanced fatigue resistance, absence of twitch potentiation, and prolonged contraction and relaxation times. These changes were associated with significant increases in Na(+)-K(+)-ATPase concentration and significant decreases in Ca(2+)-ATPase, ryanodine receptor, dihydropyridine receptor, and triadin concentrations over the course of the 20 days of stimulation. Alterations in excitability, Ca2+ handling, and excitation-contraction coupling prior to changes in myofibrillar protein isoforms may thus be responsible for early functional alterations.
为了研究慢性低频（10赫兹）刺激（CLFS）开始后力量降低的潜在机制，我们将兔胫前肌暴露于不同时长的CLFS下。为了跟踪等长收缩特性和肌电图（EMG）活动的变化，我们在10赫兹的终末测试中对受刺激肌肉和对侧肌肉进行了10分钟的研究。此外，还检测了肌浆网Ca(2+)-ATP酶的活性和蛋白量、Na(+)-K(+)-ATP酶的含量以及三联体连接成分的表达模式。刺激开始后不久，力量输出和EMG振幅迅速下降，这表明大量纤维群出现了不应性。尽管在刺激6天及更长时间后，抽搐力以及程度较轻的EMG活动逐渐恢复，但肌肉表现出了深刻改变的特性，即增强的抗疲劳能力、无抽搐增强以及延长的收缩和舒张时间。在20天的刺激过程中，这些变化与Na(+)-K(+)-ATP酶浓度的显著增加以及Ca(2+)-ATP酶、兰尼碱受体、二氢吡啶受体和三联蛋白浓度的显著降低有关。因此，在肌原纤维蛋白异构体变化之前，兴奋性、Ca2+处理和兴奋-收缩偶联的改变可能是早期功能改变的原因。
